HR: 1330h
AN: A32A-0124    [PDF]
TI: Footprint Experiments in a Model of a Complex Forest Stand (ECHO Site)
AU: Aubrun, S
EM: aubrun@dkrz.de
AF: University of Hamburg, Meteorological Institute, Bundesstrasse 55, Hamburg, 20146 Germany
AU: Leitl, B
EM: leitl@dkrz.de
AF: University of Hamburg, Meteorological Institute, Bundesstrasse 55, Hamburg, 20146 Germany
AU: Schatzmann, M
EM: schatzmann@dkrz.de
AF: University of Hamburg, Meteorological Institute, Bundesstrasse 55, Hamburg, 20146 Germany
AU: * Koppmann, R
EM: r.koppmann@fz-juelich.de
AF: Forschungszentrum Juelich, ICG II, Juelich, Juelich, 52425 Germany
AB: This study takes part to the project "Emission and CHemical transformation of biogenic volatile Organic compounds: investigations in and above a mixed forest stand" (ECHO) funded by the German atmospheric research program AFO 2000. The contribution of Hamburg University is a better understanding of the transport of biogenic emissions in the atmospheric boundary layer influenced by a very rough environment as a finite forest area. The finite forest area surrounding the Research Centre of Jlich (Germany) was modelled to a scale of 1:300 and studied in the large boundary layer wind tunnel of the Meteorological institute of Hamburg University. The model of the forest must reproduce the resistance to the wind generated by this porous environment. Using rings of metallic mesh to represent some group of trees, some preliminary tests were carried out to find the arrangement of these rings that would provide the appropriate aerodynamic characteristics for a forest. The agreement between wind and turbulence profiles measured in the wind tunnel and in the field ensured that the physical modelling of the complex forest area was realistic. The comparison of the turbulence properties of the flow, inside and above the canopy, with the literature about dense canopies showed strong similarities for the spectral densities and integral length scale profiles. Since the flow structure inside the forest area is well replicated, further measurements can be carried out on the model of the complex forest stand in order to answer questions of project partners. During field campaigns in the ECHO site, profiles of VOC concentrations are measured at three different measurement towers located in the forest stand. The origin, the trajectory and the travel time of these biogenic emissions are very important parameters for the analysis of the field data. In consequence, footprint experiments were performed in the wind tunnel by moving a release point source all over the forest area upwind of the measurement towers, and measuring which fraction of tracer-gas reached the measurement towers. These experiments gave the distribution of the probability of origin of the emissions, which are sampled at the measurement towers. The principal outcome was that the inhomogeneity of the forest (clearings and the presence of the research center within the forest) plays a disturbing role in the dispersion process of the emissions. Air masses with completely different trajectories and histories are simultaneously sampled, mixed and analyzed during field experiments. The few clearings located in the forest cause the upward deflection of air masses, which were in contact with the ground. In consequence, the contribution of biogenic emissions from the soil must not be neglected, even in the analysis of samples taken above the canopy. Some simple relationships to quickly estimate the travel time and the travel height of the air masses inside this specific forest, just knowing the wind speed at the monitoring station, were also proposed. In a next future, the vertical transport of the emissions above the forest will be studied in the physical model of this complex forest area. In order to replicate in the model the emissions released by the trees, an area source releasing gas at 80% of the tree height (location of the tree crown) will be designed and the turbulent mass fluxes above the canopy will be measured.
UR: http://www.fz-juelich.de/icg/icg-ii/echo/home
DE: 0315 Biosphere/atmosphere interactions
DE: 0368 Troposphere--constituent transport and chemistry
DE: 3322 Land/atmosphere interactions
DE: 3379 Turbulence
SC: Atmospheric Sciences [A]
MN: 2003 Fall Meeting